The present invention relates generally to biomedical implant devices, and more particularly to a method for forming biocompatible components.
A natural joint in the human body such as a knee joint may undergo degenerative changes due to a variety of etiologies. When these degenerative changes become advanced and are irreversible, it may ultimately become necessary to replace the natural joint with a prosthetic joint. Such a prosthetic joint often includes several biocompatible components which are formed from high strength synthetic materials. These materials are not only able to accommodate the various loading conditions that the prosthetic joint may encounter, but are also biocompatible with the human body. An example of such high strength synthetic materials is ultra-high molecular weight polyethylene which is often used when there is relative movement between the adjacent metallic surface of a prosthetic joint.
Biocompatible components which are made from ultra-high molecular weight polyethylene are often formed using one of two different techniques. In one technique, a relatively precise amount of polyethylene powder is placed between two halves of a die which are then simultaneously compressed and heated. After the powder is densified using standard sintering techniques, the die is allowed to cool. The biocompatible component is then removed from the die and is sterilized in a manner well-known to those skilled in the art.
In the second technique, a substantially completely consolidated polyethylene stock is first formed and then the biocompatible component is machined from the substantially completely consolidated stock. Several methods exist which may be used to form the substantially completely consolidated stock. In one method, the substantially completely consolidated stock is extruded by placing polyethylene powder in a cylindrical chamber having an opening of a particular shape at one end of the chamber. A hydraulically operated piston located at the other end of the cylinder is then used to compress the polyethylene powder. The force exerted by the piston on the polyethylene powder causes the powder to compact. Heat is also applied to solidify the powder as it moves through the cylinder. In another method for forming a substantially completely consolidated stock, polyethylene powder is placed between two flat plates which are compressed while heat is applied. As this occurs, the polyethylene powder is densified so as to form the substantially completely consolidated stock.
While these two techniques for forming biocompatible components are effective, they nevertheless have certain disadvantages. With respect to the first technique described above, it will be appreciated that only one biocompatible component can be made at one time. Accordingly, this technique is relatively inefficient in terms of the amount of time required to make the biocompatible component. With respect to the second technique in which the biocompatible component is formed from a substantially completely consolidated stock, the resulting consolidated stock may often require a stress relief operation or an annealing operation prior to machining. In addition, when polyethylene stock is formed by heating polyethylene powder between two plates acting under pressure, the resulting may have density gradients or voids due to the relatively nonuniform pressure applied to the powder across the plates.
In addition, methods are also known for treating ultrahigh molecular weight polyethylene prior to being machined into a biocompatible component. One such method is disclosed in U.S. Pat. No. 5,037,928. However, during the procedure described in this reference, the polyethylene stock is placed under a sufficient pressure so as to induce pressure crystallization of the stock. This pressure crystallization tends to cause increased susceptibility to wear. In addition, the use of this relatively high pressure required that relatively expensive pressure containment vessels be used. Furthermore, this method describes processing preformed polyethylene stock which often has unwanted density gradients or voids as described above.